diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -11592,6 +11592,9 @@
       }
 
       if (const auto *FD = dyn_cast<FunctionDecl>(PV->getDeclContext())) {
+        // Skip function template not specialized yet.
+        if (FD->getTemplatedKind() == FunctionDecl::TK_FunctionTemplate)
+          return;
         auto ParamIter = llvm::find(FD->parameters(), PV);
         assert(ParamIter != FD->param_end());
         unsigned ParamNo = std::distance(FD->param_begin(), ParamIter);
diff --git a/clang/test/SemaTemplate/decltype.cpp b/clang/test/SemaTemplate/decltype.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaTemplate/decltype.cpp
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -verify %s
+// no crash & no diag
+
+// expected-no-diagnostics
+template <typename T>
+auto foo(T x) -> decltype((x == nullptr), *x) {
+  return *x;
+}
+
+void bar() {
+  foo(new int);
+}